The present exemplary embodiments relate generally to raster output scanners. In one particular application, raster output scanner sensitivity to banding is mitigated by reducing sensitivity of error caused by emitter rotation. It is to be appreciated, however, that the present exemplary embodiment is also amenable to other like applications.
A conventional multi-beam raster output scanner (ROS) has a reflective multifaceted polygon mirror that is rotated about its central axis. The polygon mirror repeatedly sweeps beams of light emitted from a modulating laser light source across a photoreceptor. The photoreceptor can be on a drum that rotates about an axis or on a belt that rotates along a closed path on rollers. In either case, the beams move in a line scanning direction while the recording medium advances in an orthogonal (process) direction. The beams scan the recording medium in accordance with a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with a picture element (pixel) data stream. Thus, the photoreceptor is exposed to form the image represented by the pixel data.
The use of the rotating polygon mirror optical system, however, presents several inherent problems. Bow and wobble of the beam scanning across the surface of the photoreceptor can result from imperfections in the optics, the mirror and/or mechanical defects that cause instability in the rotation of the polygon mirror. These problems typically require complex, precise and expensive optical elements between the light source and the rotating polygon mirror and the surface of the photoreceptor. Optically complex elements are also needed to compensate for refractive index variation, wherein the refractive index varies for different portions of the optics that image to different points on the photoreceptor. If the focus is different, the writing spot can be a different size or displaced from an intended position that causes changes in the focal length of the imaging optics of the ROS.
In some embodiments, the modulating laser light source consists of a vertical cavity surface emitting laser (VCSEL) array. The VCSEL array may be either a one- or two-dimensional array of individual laser sources. Each individual laser source in the VCSEL array has a corresponding drive circuit which may be used to generate a beam to expose a corresponding area on the surface of the photoreceptor in response to video data information applied to the drive circuits of the VCSEL array. The photoreceptor is advanced in the process direction to provide a desired image by the formation of sequential scan lines generated by the beam-to-beam exposure delivered from the VCSEL array. As utilized herein, an array with a vertical orientation has a greater number of rows than columns of light emitters. In contrast, an array with a horizontal orientation has an equal or lower number of rows than columns of light emitters.
The chief deficiency of a high resolution VCSEL Raster Output Scanner is the sensitivity of banding to very small amounts of emitter array rotation. While overwriting can suppress some types of banding, it does not suppress banding due to emitter array rotation error for conventional emitter geometry. This lack of suppression occurs because overwriting writes the rotation exposure error of one pass exactly on the rotation exposure error of the previous pass. Hence, the exposure error is not reduced by averaging over two exposures. Additionally, current error correction methods write row-to-row seams on swath-to-swath seams, thereby increasing the amplitude of banding due to those interacting seams.
FIG. 1 provides an example of the rotation error using an array 100 in a vertical orientation, which contains thirty-two light emitters arranged in four columns and eight rows. Accordingly, the array 100 is a vertical orientation, as it has a greater number of rows than columns. In this example, the array has a slight counterclockwise rotation error that leads to a raster spacing error, wherein the spacing of the rasters between the rows is greater than the spacing within the rows. Also, the seam between successive swaths can be seen to lie on the seam between rows, which can further exacerbate the exposure errors.
FIG. 2 illustrates rotation error caused for an array 200 with a horizontal orientation. In this embodiment, the array 200 is rotated ninety degrees from the array 100 to enable greater optical efficiency needed for high speed image-on-image products. The horizontal orientation, however, is more sensitive to banding because the row length is longer (e.g., by a factor of 2-3) and the frequency of the rotation error is halved over the first geometry 100. Accordingly, what are needed are systems and methods to mitigate banding in raster output scanners regardless of VCSEL array orientation.